The use of nasal CPAP apparatus to treat “snoring sickness” was pioneered by Sullivan and taught in U.S. Pat. No. 4,944,310. Nasal CPAP apparatus typically comprises a blower, an air delivery conduit and a patient interface. The blower provides a supply of air or breathable gas at positive pressure. The conduit interconnects the blower and the patient interface. A variety of nasal masks, nose & mouth masks, full face masks, nasal prongs and nasal pillows are used to provide an interface with the patient.
A typical mask comprises:    (i) a rigid or semi-rigid portion, termed a shell or frame, which defines a nose-receiving cavity; and    (ii) a soft patient contacting portion, termed a cushion or membrane.Cushions have been constructed from silicone, foam, gel and combinations of these materials.
Since a patient typically exhales into the same mask cavity wherefrom they inhale, the possibility of rebreathing of carbon dioxide (CO2) exists. In conjunction with a sufficient continuous flow of fresh air or breathable gas, a vent can allow a controlled leak from the mask cavity and hence provide for the washout of CO2. Unfortunately, the noise of air or breathable gas from the vent can disrupt anyone within earshot attempting to sleep. Hence there is an advantage in providing a low-noise vent.
One form of known vent is described in U.S. Pat. No. 6,561,190 (Kwok) and U.S. Pat. No. 6,561,191 (Kwok). These patents describe the use of grommet in a mask frame. The contents of these patents are hereby incorporated by cross-reference. A vent in accordance with embodiments of these inventions is found in the MIRAGE™ mask, manufactured by ResMed Limited.
Another known form of vent is described in International Patent Application PCT/AU00/00636 (Drew et al.) published as WO 00/78381. This patent application describes the use of a connector for a mask having a vent along a smooth continuing surface. The contents of this patent application are hereby incorporated by cross-reference. A vent in accordance with an embodiment of this invention is found in the ULTRA MIRAGE™ mask, manufactured by ResMed Limited.
Another known form of vent is described in U.S. Pat. No. 6,581,594 (Drew et al.). This patent describes the use of a vent which, in one form, comprises a thin air permeable membrane. The contents of this patent application are hereby incorporated by cross-reference.
Another known form of vent is described in International Patent Application PCT/AU01/01658 (Dantanarayana et al.) published as WO 02/051486. This patent application describes the use of a flow regulation vent. The contents of this patent application are hereby incorporated by cross-reference.
U.S. Pat. No. 6,557,555 (Hollis) describes a vent valve apparatus. The contents of this patent application are hereby incorporated by cross-reference.
Another known vent is the Respironics WHISPER swivel.
European Patent No. 0 697 225 discloses a vent formed from a porous sintered material.
A known vent, manufactured by Gottleib Weinmann Geräte Für Medizin Und Arbeitsschutz GmbH and Co. comprises a generally cylindrical insert to be interposed in use, between the mask shell and the gas conduit. The insert includes a window which is covered with a porous sintered material of approximately 3-4 mm thickness.
Another type of vent intended to be inserted between the mask shell and the breathable gas supply conduit is the E-Vent N by Draeger medizintechnik GmbH (the Draeger vent). The Draeger vent comprises a stack of 21 annular disks, which have slots in their adjacent surfaces for gas to flow therethrough. Each slot has a length of 5 to 7 mm as measured along the path from the interior of the vent to atmosphere.
Typically vents are designed with sufficient porosity to provide enough vent flow at a low pressure (e.g. 4 cmH2O) to ensure adequate washout of CO2.
Reducing the pore size of a vent can make the vent quieter, but can also increase the chances that the vent will clog.
Problems with prior art vents include that they can be too noisy, that they clog with dirt and moisture (particularly when used with humidifiers), that they are awkward or difficult to clean or assemble and that they have designs which are sensitive to very small changes in the manufacturing process which can lead to variation in the pressure flow relationship.